1. Field of the Invention
The present invention relates to a semiconductor device with an overvoltage protective function and a method of fabricating such device, and more particularly to a semiconductor device having a better accuracy in the breakdown voltage of an overvoltage protective mechanism such as power thyristors and IGBTs handling a high voltage and to a method of fabricating such device.
2. Description of the Related Art
High capacity semiconductor devices for converting a high voltage, to which an overvoltage several times as large as a rated voltage thereof is applied, are often broken down. Accordingly, semiconductor devices containing an overvoltage protective function have been developed. Prior art will be explained taking thyristors with overvoltage protective function as an example. Overvoltage protective function is broadly classified into punch-through type and avalanche type.
Thyristors with punch-through type voltage protective function are described, for example, in the literature, Laser Trimming of Thyristors, IEEE PESC, '85, pp 463-468. In the punch-through type, a recess is provided on a P gate base layer. When a forward blocking voltage (forward voltage in off state) is applied to a thyristor, a depletion layer is formed and spread near the junction of the P gate base layer and an N base layer, with the increase of the forward blocking voltage. When the forward blocking voltage is further increased to reach a breakdown voltage V.sub.BD, at which self protective function is performed, the depletion layer on the P gate base layer side reaches the recess to be punched through. This causes a flowing current to function as the gate current of a pilot thyristor and turn on the pilot thyristor, so that the amplified on-current of the pilot thyristor becomes the gate current of a main thyristor, whereby the main thyristor is safely turned on to cause the thyristor to be protected. The self protective breakdown voltage V.sub.BD is selected to set a proper value less than the minimum overvoltage V.sub.BDM which might break down the thyristor.
In the thyristors with a punch-through type over-voltage protective function, for example, the thickness of the depletion layer extending to the P gate base is very narrow compared with that extending to the N base due to the concentration of the impurity in the respective base layer, so that the accurate control of the self protective breakdown voltage V.sub.BD in the course of fabrication is very difficult with respect to mass productivity even if it can be performed in a laboratory step.
Thyristors with avalanche type voltage protective function are described, for example, in the literatures, Photothyristors with Overvoltage Protective Function, ED85-4, pp. 23-29 Toshiba; Basic Characteristics of High-Voltage Withstand Self Protective Type Photothyristors, EDD-86-53, pp. 69-75, Hitachi; and Controlled Turn-on Thyristors, IEEE, Trans-Electron Devices, ED-30, pp. 816-824 (1983) GE.
In the avalanche type thyristors, provided on a part of the P gate base layer of a PNPN structure thyristor is a region in which avalanche breakdown is more liable to occur than in other parts. By the rising transient voltage V.sub.BD (self protective breakdown voltage) of a destructive overvoltage, first the region is avalanche broken down, then a non-destructive avalanche current flows to the P gate base layer of a pilot thyristor, whereby the pilot thyristor and subsequently a main thyristor are turned on to attenuate the overvoltage, causing the thyristor to be protected.
The avalanche type thyristor is provided with a crook on the trigger light incident surface of the center thereof or on the junction directly under the gate electrode of the pilot thyristor. As known, the crook, on which electric field is concentrated, is more liable to occur have avalanche breakdown occur than other parts. A desired self protective breakdown voltage V.sub.BD is obtained by changing the curvature of the crook.
In the above conventional avalanche type thyristors, the gate base region, in which avalanche breakdown is performed, is formed in the course of element fabricating process, so that when the self protective breakdown voltage V.sub.BD is measured upon the completion of the element, the breakdown voltage indicates inevitably varied values because of variation in material or process. Since the avalanche voltage has a positive temperature dependence, inevitably the breakdown voltage also has a temperature dependence. This means that the breakdown voltage at a high temperature becomes higher than at a normal temperature, so that the design of thyristors becomes severe, in particular, with respect to voltage-withstand and di/dt quantity-withstand.